Aside from, well power chords, what other chords sound good played with gain?

You're not going to like this answer, but, any of them. Chords, like all notes, and moments in musical time, are contextual, and depending on what lies immediately before or after them, can change dramatically the emotional impact of any other adjacent moments, and thus any chord can be used properly given the proper conviction (and, of course, given you've experimented enough to find what it is you like).

Even major chords cannot be ruled out. Minor keys are of course, the foundation for most metal, but chord-scales utilize major and minor chords in an alternating fashion. In the Aeolian mode for instance, the Dominant V should be a Major chord. Play A minor, then E major, then return to A minor. The E chord still sounds very dramatic, but utilizes all of the notes in the A major scale, so as long as we at some point return to the tonic, we should be at some point pleased with our results after wandering around for a little while.

Yeah, the question really is "what chords work with distortion".
Power chords of course, but they aren't technically chords, so...
I often use arrangements where one guitar plays a power chord and other one plays root and third, so together they make the whole chord without distortion messing it up. Or sometimes even, especially in black metal where the same (power) chord can last a bar or even longer, have the bass guitar to visit the third momentarily to give a stronger feeling of the chord.
Usually tight triads get muddy with distortion, but spreading them out helps. So instead of root-third-fifth a version a chord where the third is over octave from the root can work better. Especially with sus-chords, instead of sus2 a sus9 (is that a real term? Anyway, root-fifth-ninth) is much more clear. And that sus9 is a pile of two power chords which lessens the dissonance generated by distortion.
I've never gotten seventh chords sound good with distortion, they just get too dissonant. Although, those who grew up during the 80's could be more used to a bit harsh sounding "nintedo-chord", or root-third-minorseventh played with gritty square waves. Games like Super Mario Bros and Mega Man are full of them.

the question I think you want to ask is "what modes or scales" work for metal...

phrygian
phrygian dominant
minor
melodic minor
harmonic minor
pentatonic
lydian
mixolydian
diminished

I'd start with these.

The short, flippant answer: Buy a chord chart/book and make your own decisions.

The longer answer: Any of them will work depending on context and your goals. However, a good guideline to remember is that when adding distortion, smaller intervals (ie, how far apart the notes are in pitch) will sound muddier. This compounds when you play in the lower register of the guitar. Having tight intervals such as 3rd and 2nds will require a certain amount of gain staging, attention to picking attack and good intonation on your guitar. Consider rolling the volume knob on your guitar back a bit if you want to make thick, colourful chords sounds less muddy. Also consider that chords can be broken into individual notes in order to soften the density of the harmony. Also consider that the same chord can be voiced in many different ways and some of these voicings will sound better than others. Try not to quit when you realize that Father Time will see dead in the ground before you know everything there is to know about chords and harmony.

If you want to learn more about chords but don't know where to start, research MAJOR and MINOR chords/triads. Most harmony uses these forms as a starting point. There are a million books and five million YouTube tutorials on this.

Have fun.

The root and 5th of a power chord shape and a C minor chord shape are good shapes to use for the chords in your song. They are easy to move around and pick fast for black metal or death metal. Just Use your bass parts to add a 5th, 7th, lower octave root, or whatever works to add some more depth. The chords themselves don't matter too much if you know how to work with your notes to make them fit a certain mood. Look at some charts online comparing major scale to minor scale to major 7th, minor 7th, and so on. They each raise/lower certain notes to put the scale in an entirely different mood and you essentially just need to make sure your chord shapes fit within those notes.

I like Hirajoshi scale